In recent years, projection type displays, such as liquid crystal projectors, have been being used widely. Especially, there are demands for projection type display apparatuses having capability of daytime use or use without turning off interior illumination. Therefore, it is demanded that a light source itself arranged in the projection type display apparatus be brighter, and have good efficiency. As such a light source, a short arc type extra-high pressure discharge lamp which contains mercury inside its electrical discharge space, and which continuously emits high intensity light in the visible light range due to a very high pressure at time of lighting is widely used.
In such extra-high pressure discharge lamps, there are a direct-current lighting type and an alternating current lighting type. As a direct-current lighting type cathode or an alternating current lighting type electrode, a melted electrode in which a coil-like member is inserted onto the tip of a rod shape-member which is made from tungsten material, and the tip thereof is melted by electric discharge etc., is used widely.
However, since it is difficult to stably form the shape thereof when melting the tip portion of the melted electrode at the time of manufacture, an electrode produced by cutting work was proposed, and has been reduced to practice in some areas. Such an extra-high pressure discharge lamp and an electrode for an extra-high pressure discharge lamp are disclosed in, for example, Japanese Patent No. 3,623,137.
In FIG. 7, a conventional extra-high pressure discharge lamp and an electrode arranged in the conventional extra-high pressure discharge lamp, are shown. FIG. 7 is a schematic cross sectional view showing the structure of the conventional extra-high pressure discharge lamp 51. This extra-high pressure discharge lamp 51 has an electric discharge container 52 which is made of quartz glass, a pair of electrodes 53 whose tips are arranged so as to face each other in the electric discharge container 52, metallic foils 54 welded to the respective electrodes 53, and external lead rods 55, each of which is welded to the other end of the metallic foils 54. Moreover, sealing portions 56, each of which is formed by bringing part of the electrode 53, the metallic foil 54, and the external lead rod 55 into close contact with glass, are formed. The electrodes 53 are made from tungsten material. A tip portion 53a of each electrode 53 having a large outer diameter, and an axis portion 53b having a small outer diameter connected to the tip portion 53a are formed on the electrode by cutting work. Moreover, the axis portion 53b is made up of an embedded portion 53c buried so as to be surrounded by the glass material of the sealing portion 56, and a projection portion 53d which projects in the electric discharge container 52.
When carrying out cutting work on the electrode 53, in the conventional processing method, one end of the electrode material made from rod shape tungsten material is held, and using a numerical control lathe (NC lathe) etc., a chip for cutting is pressed onto an outer circumference surface of the electrode material while rotating it, and the chip for the cutting is moved in an axial direction of the rod shape tungsten material. Thus, minute unevenness (cutting marks) approximately in a direction perpendicular to the electrode axial direction is formed over the entire electrode surface of the processed electrode.
In the conventional extra-high pressure discharge lamp, cracks are generated in the sealing portions formed by bringing the electrode into close contact with the glass, and there is a problem that the extra-high pressure discharge lamp itself is broken in some cases. This phenomenon appears more notably as the contact area of the electrode and the glass is larger. This attributes to stress which is generated in the glass since the difference of thermal expansion coefficient is generated between the expansion contraction of the electrode and the expansion contraction of the glass in close contact with the electrode when the extra-high pressure discharge lamp repeats light-on and light off.
A measure to such cracks is known, as disclosed in, for example, Japanese Laid Open Patent No. H11-176385. The Laid Open Patent discloses the technology of preventing generation of cracks by inserting a coil-like member in the sealing portion which is formed so that the electrode may be in close contact with the glass and making the close contact area of the electrode and the glass small, so as to ease the stress generated in an interface with glass. However, although the entire lamp comes to be exposed at a higher temperature as an output of the extra-high pressure discharge lamp itself is higher, the problem of cracks has not been fully solved only by the conventional technology, so that there is a problem that reliability cannot be obtained as the extra-high pressure discharge lamp. Moreover, with demands of the market, while developments of lamps according to much higher pressure power specification, which are lamps with high light emission efficiency, progress, fine cracks which have not been considered by now, become problematic as a factor of breakage. Moreover, since the reliability over breakage-proof was not enough, there was a problem that the extra-high pressure discharge lamp with a long-life span could not be produced.